Collinear array dipole antennas are well known for providing omnidirectional radiation. The prior art includes antennas such as the Franklin antenna, schematically illustrated in FIG. 1, the series-fed transposed coaxial collinear antenna of FIG. 2, and the series-fed symmetrical coaxial collinear antenna shown in FIG. 3. Current distribution along the array is graphically depicted for each antenna. These antennas inherently possess a narrow bandwidth. This is because the radiating elements are series fed, resulting in varying transmission phase lengths from the array feed point to the various dipoles of the array. A dipole array which is parallel fed, having equal transmission line feeds from the common array feed point to each dipole, will undergo a similar phase shift to each dipole as frequency is varied. The result is a more uniform radiation pattern over its bandwidth. A common method of feeding stacked dipole antennas in parallel is to side mount dipoles off a central support structure, spacing them symmetrically around and close to the mast at 90 degree increments as shown in FIG. 4. This is to minimize the deviation from circularity in the azimuth of each dipole. The support mast actually is a parasitic element in this configuration, and results in a cardiod pattern for each dipole location. The "phase center" of the dipole/mast structure is located along a line between the mast and the dipole, and hence, the phase centers of the various dipole antenna locations are not axially aligned, or collinear. This results in a pattern that deviates from circularity by typically +/-1.5 dB for a nominal 6 dB gain antenna. In addition, the center of the main lobe will deviate above and below the horizon to some degree, as one views the pattern from various sectors in the azimuth.
As a result of the preceding problems, with rare exceptions, elemental center-fed dipole antennas are not used in vertical polarization applications due to mounting and feeding effects on symmetry. The symmetry problem is partially alleviated through the use of series fed symmetrical coaxial collinear antenna arrays such as illustrated in FIG. 3. Such arrays are comprised of coaxial center fed halfwave dipole elements with a choke as illustrated in FIGS. 5A, B and C where an antenna is illustrated structurally and schemati-cally. However, such arrays suffer the same disadvantage as all other known vertical arrays of dipole antenna elements in that they require a precise relationship between radiator element spacing and radiator element length, resulting in a narrow bandwidth which precludes their use in broad band applications.